Major doubts also hang over the participation of James McCarthy and Daryl Murphy due to hamstring and calf problems respectively.

O'Neill would generally lean towards the tried and trusted for a game of this magnitude and does have enough senior internationals to field an experienced starting XI.

However, the loss of key men means that his bench could be stacked with individuals who have little or no competitive experience for their country.

If centre-halves Richard Keogh and John O'Shea were to suffer a setback, O'Neill would have to turn to Alex Pearce - who has never featured in a qualifying game - or the uncapped John Egan and Andy Boyle.

And if he wants to change his midfield during the match then he would have to turn to fringe squad members Eunan O'Kane, Callum O'Dowda, Jonathan Hayes or new faces Daryl Horgan and Conor Hourihane. They are all one injury away from coming into serious consideration.

O'Neill says the identity of the opponent could make it easier to pitch in fresh faces because they will be aware of the Welsh squad from club business.

"Perhaps throwing someone in at the deep end might not be as severe considering they should know the players a wee bit better than maybe going to a game in Belgrade or something like that," said O'Neill.

Captain Seamus Coleman and Hayes sat out training yesterday with minor knocks while the picture on McCarthy and Murphy should become clearer in the next 24 hours.